Port to ECMAScript 6 (version 2.10.0)

Moment 2.10.0 does not bring any new features, but the code is now written in
ECMAScript 6 modules and placed inside src/. Previously moment.js, locale/*.js and
test/moment/*.js, test/locale/*.js contained the source of the project. Now
the source is in src/, temporary build (ECMAScript 5) files are placed under
build/umd/ (for running tests during development), and the moment.js and
locale/*.js files are updated only on release.

If you want to use a particular revision of the code, make sure to run
grunt transpile update-index, so moment.js and locales/*.js are synced
with src/*. We might place that in a commit hook in the future.

We're looking for co-maintainers! If you want to become a master of time please
write to ichernev.

In addition to contributing code, you can help to triage issues. This can include reproducing bug reports, or asking for vital information such as version numbers or reproduction instructions. If you would like to start triaging issues, one easy way to get started is to subscribe to moment/moment on CodeTriage.